Hyaluronic acid exists in animal tissues and has biocompatibility and biodegradability. With regard to physical properties, it has superior properties such as a highly water-absorbing property, and the aqueous solution thereof has high viscosity.
A hydrogel is obtained by chemically modifying hyaluronic acid, crosslinking the modified hyaluronic acid by some methods to form a network structure, and incorporating an aqueous medium such as water into the network structure. The hydrogel shows viscoelasticity as well as viscosity.
The crosslinked hyaluronic acid apparently forms macromolecules irrespective of the bonding mode. The biodegradability of the crosslinked hyaluronic acid can be controlled by adjusting the degree of crosslinking.
There are various crosslinking modes. For example, as a crosslinking mode making use of a hydrophobic bonding or an ionic bonding, a crosslinking of hyaluronic acid by introducing a nucleophilic reagent thereto (JP-W-3-502704 (corresponding to U.S. Pat. No. 4,937,270), the term "JP-W" as used herein means an "unexamined published international patent application"), a crosslinking via a hydrophobic bonding by esterification of hyaluronic acid (U.S. Pat. No. 4,851,521), and a crosslinking via an ionic bonding by polyvalent ions (EP 0507604 A2) have been known. Since they are crosslinked by a weak bonding force compared to that of a covalent bonding, they are susceptible to influences of external changes such as pH, ionic strength, temperature and the like. In addition, when they are used as biomedical materials, remainability in a living body is short, and it is difficult to control properly remainability in the body so as to maintain the physiological effects of hyaluronic acid for the body.
Furthermore, as a crosslinking mode of binding of hyaluronic acid molecules by a covalent bonding, a crosslinking via divinylsulfone (JP-B-4-30691 (corresponding to U.S. Pat. No. 4,582,865), the term "JP-B" as used herein means an "examined Japanese patent publication") and a crosslinking via an epoxide (JP-W-61-502729 (corresponding to U.S. Pat. No. 4,886,787), JP-A-5-140201, the term "JP-A" as used herein means an "unexamined published Japanese patent application") have been known. However, the crosslinking agents or the crosslinking compounds used in these crosslinkings are toxic. Besides, a three-dimensional network structure is constituted by crosslinking at the same time when divinylsulfone, epoxide or the like is introduced into hyaluronic acid, and the formed crosslinked hyaluronic acid gel is insolubilized in a solvent such as water and the like. Unreacted low-molecular compounds thus incorporated into the network structure are difficult to separate and remove.
On the other hand, a crosslinking of hyaluronic acid by a photocured crosslinking reaction through irradiation with ultraviolet rays (JP-A-6-73102 (corresponding to U.S. Pat. No. 5,462,976), ASAIO Journal, 38, M154-M157 (1992)) has also been known. This crosslinking mode has the advantages that a photoreactive hyaluronic acid derivative into which a photoreactive crosslinking group is introduced is water-soluble before photocured crosslinking and a three-dimensional network structure is not formed at the time so that unreacted low-molecular compounds can be removed with ease; the photoreaction itself is such a clean reaction that yields a photocured crosslinked-hyaluronic acid derivative free from unreacted low-molecular compounds; and the resulting crosslinked structure is formed by a covalent bonding so that the control of the remainability of the photocured crosslinked-hyaluronic acid derivative can be easily performed by controlling a degree of crosslinking.
Also, when the above-described photocured crosslinked-hyaluronic acid derivative was intended to apply for uses as biomedical materials, e.g., antiadhesive materials, those in the film-like form had been investigated (ASAIO Journal, 38, M154-M157 (1992)), but it has been difficult to prevent adhesions in fine parts of tissues or organs. Thus, an injectable photocured crosslinked-hyaluronic acid gel which can be injected into such a fine site has therefore been demanded.
However, hydrogel of a photocured crosslinked-hyaluronic acid and methods for preparing the same as disclosed in the present invention have been unknown.
A conventional crosslinked hyaluronic acid hydrogel has difficulty in removing impurity such as unreacted low-molecular compounds and the like and controlling the physical properties of the hydrogel. Furthermore, it has been difficult to establish conditions for preparing hydrogel having desired physical properties.